Network and premises communications systems--even robust ones--are typically engineered to handle the so-called "average busy hour" traffic load because it is uneconomical to design communications systems to handle worst case "peak" loads. Thus, when the arrival rate of calls is beyond the average busy hour system capacity, calls are simply blocked resulting in the application of a busy tone to callers' telephone sets.
To reduce incidences of call blocking, large inbound telecommunications service subscribers usually lease bulk facilities, such as DS-1 lines that connect those subscribers' premises equipment to a communications carrier's terminating switch using a common interface such as, the Primary Rate Interface (PRI) of the Integrated Services Digital Network (ISDN) standards. A call directed to those inbound subscribers is blocked when the terminating switch determines that no circuit is idle in the leased facility.
Blocked calls directed to inbound telecommunications service subscribers result in economic losses for all parties involved. Callers are frustrated by their inability to communicate with the subscribers (or their representatives) to complete or inquire about commercial transactions of interest. From the subscribers' perspective, blocked calls represent potential losses of business. A more concrete economic loss is the uncompensated use of the switching and transmission facilities of the carrier or carriers that attempted to complete the blocked calls. Adding further economic injury is the fact that Interexchange carriers (IXC) incur access and/or egress charges for blocked calls. The problem is further compounded when repeated call attempts from persistent frustrated callers result in repeated blocked calls.